Aerospace vehicles utilize venting systems to equalize the pressure between the external environment and various internal cavities. Aircraft fuel systems, for example, are fitted with one or more ventilation ports configured to maintain fuel tank pressure within design limits throughout the operating flight envelope. Ventilation is particularly critical during steep climbs and descents where ambient pressure changes rapidly, potentially imposing a pressure gradient across tank walls. Design of the ventilation port itself is complicated by the fact that the port should typically act as a reasonably efficient nozzle under flight conditions where fuel system is under positive pressure, but should also act as a reasonably efficient inlet when the tank is under negative pressure.
Existing ventilation ports generally take the form of an inlet, often a flush scoop. These scoops are specifically intended to bring ambient air into a system with minimal impact on the ambient flow field. Thus, they pressurize the fuel system during descent when the tank is under suction pressure (e.g., as required during descent). However, the dynamic pressure head generated by such inlets acts to retard ventilation flow out of the system when tank pressure exceeds ambient pressure (e.g., as required during climb).
Because the inlet brings air onboard nearly parallel to the local external flow direction, ducting is required to turn the flow into the tank. This ducting requires volume, and increases system weight and cost.
Other prior art ventilation systems use flush holes. Such holes are advantageous in that they reduce system volume and weight because the vent port can feed the tank directly without the need for routing ducts. Relative to a flush inlet, a flush hole trades degraded performance while bringing air into the fuel system to improve system efficiency when purging air out of the fuel system, such as would occur as the aircraft climbs. This system also reduces the peak positive pressure that the fuel tank needs to accommodate, providing an additional opportunity to reduce structural weight. However, flush holes are notorious for generating tonal whistles, known as organ pipe or Helmholtz resonances, when the ventilation flow rates are low.
Prior art venting systems are thus undesirable in a number of respects. For example, typical ventilation ports do not permit tank pressures to neutralize quickly, and therefore necessitate increased aircraft weight as the fuel system tanks and ducting are strengthened to withstand a broader range of operating pressures. In addition, there is a need for venting systems that fit compactly within the aircraft to maximize flexibility in placement while minimizing the weight and cost of any required ducting. There is also a need for duct systems that result in minimal disruption of the ambient flow, thus providing the highest permissible aircraft aerodynamic performance. It is also desirable that duct systems be passive rather than active in nature (to minimize cost and maximize system reliability), and to avoid collateral impacts such as acoustic resonance and the like. Furthermore, inlet-configured ports are biased to pressurize the tank. They also require ducting to turn vent flows inside the mold-line, thus adding weight.
Accordingly, it is desirable to provide improved venting systems for venting fuel tanks and other such internal cavities. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.